In the manufacture of bevel and hypoid gears it is known to utilize circular face mill type cutters, including form-relieved face mill cutters, having a plurality of cutting blades extending in an axial direction from one side of a cutter head. Each cutting blade includes a front face, top (or tip) surface, cutting (pressure) side surface, cutting edge, clearance side surface and a clearance edge. A protuberance surface may also be included usually near to the top surface. In form-relieved face mill cutters, the side surfaces and the top surface are helicoids. When the front face of a cutting blade is removed (e.g. ground) for sharpening purposes, the new front face profile has the same shape and radial position relative to the cutter axis as the prior profile, but is displaced axially toward the back (or bottom) surface of the cutter.
The cutting blades of a face mill cutter may be of the type known as outside cutting blades which cut the concave side of the teeth of a gear workpiece, or the cutting blades may be of the type known as inside cutting blades which cut the convex side of the teeth of a gear workpiece. Face mill cutters may also have alternatively arranged inside and outside cutting blades about the cutting tool. These alternating blade type of cutters form the entire tooth slot between adjacent teeth on a workpiece since each pair of inside and outside cutting blades forms the opposite sides of adjacent teeth. The cutting blades of a face mill cutter may be separate from the cutter head and attached thereto via means such as bolts, or the cutting tool may be of the type known as a “solid” cutter with cutting blades and cutter head formed integral with one another (i.e. one-piece) and produced from material such as hardened tool steel (e.g. APS 2030) or carbide (e.g. H10F).
In the grinding of tooth profiles of solid bevel gear form-relieved cutting tools a plurality of grinding wheels and/or a plurality of grinding machines have been used to complete the entire blade profile on both side surfaces, tip surface and the front cutting face. For example, the pressure (cutting) side surfaces and the clearance side surfaces may be ground with pencil-shaped tapered cone grinding wheels having relatively small diameters in comparison to length. However, due space limitation constraints dictated by things such as cutter diameter and blade-to-blade spacing, it is usually necessary for the diameter of a grinding wheel for grinding inner blade surfaces to be smaller than the diameter of a grinding wheel for grinding the outer surfaces of the cutting blades. Smaller diameter grinding wheels are necessary for inner blade surfaces so as to avoid interference with adjacent cutting blades that would occur if the larger diameter grinding wheel for outer surfaces were used for inner surface grinding. The smaller diameter grinding wheel may also be used for grinding the outer surfaces of the cutting blades but this generally results in a slower and less efficient process.
Additionally, the skilled artisan will understand that in order to maintain a certain desired grinding surface speed (e.g. 5,000-6,000 surface feet per minute (SFPM)), it is necessary to rotate the smaller diameter grinding wheel at a higher rate than the larger diameter outer surface grinding wheel. For example, a 0.3 inch diameter grinding wheel may be rotated at 72,000 revolutions per minute (RPM) while a grinding wheel with a diameter of 2.0 inches is rotated at 10,000 RPM in order to keep the grinding surface speed in the range of 5,000-6,000 SFPM.
For grinding the front cutting face, a flared cup-shaped grinding wheel with a flat grinding portion may be used to produce the compound face angle with offset rake and hook. Bevel gear solid cutters in the range from about 0.5-6.3 inches in point diameter limit the size of the flared cup-shaped grinding wheel for face sharpening by diameter interferences. The flared cup-shaped grinding wheels usually range from about 2.0 inches to about 7 inches diameter to eliminate interferences and maximize material removal rates. As the size of the flared cup-shaped grinding wheels increases the rotation speed decreases from about 16,000 revolutions per minute for the smallest diameter to about 3,000 revolutions per minute for the largest diameter to maintain a preferred wheel surface speed for face sharpening in the range of 3,000 to 4,000 surface feet per minute.
The top surface of the cutting blades may be ground by including an appropriately dressed shoulder portion formed on one or both of the inner or outer pencil-shaped tapered cone grinding wheels. However, the addition of this feature to the inner/outer grinding wheels further enhances the likelihood of interferences. Additionally, as the cutting blade size increases, the use of larger top flats and/or radii is predominate. In many cases, it is preferred to utilize a separate cup-shaped grinding wheel in order to provide an adequate top flat surface, tip chamfers, and faceted pre-contoured large tip radii.
While the utilization of separate grinding wheels for grinding different cutting blade surfaces overcomes the problem of interferences, a disadvantage is adding processing time with separate grinding operations. Some current tool manufacturing practices use the blending of profiles produced from multiple grinding wheels and wheel shapes to grind the tooth flanks, tooth tip radii, and tooth tip flat in multiple (e.g. up to five) setups. With CNC machine motion technology and rotary truing/dressing devices it is possible to contour the above mentioned pencil-shaped, flared or cup-shaped grinding wheels to incorporate multiple features, for example, the blade bottom radius/ramp, blade pressure angle flank, top rim, blade tip radius, and blade tip flat. In the example mentioned, the finish grinding process may be reduced to blending profiles of just two pencil-shaped grinding wheels. Probing of profiles, utilizing probes such as the Renishaw 3-D and with acoustical touch sensing, assists in the relative positioning of the grinding wheel to the inside and outside blade reference points and blending of profiles to achieve the required blade profiles, blade thickness, blade height, and bevel gear cutter point diameter.
Given the above, many tool manufacturing facilities employ a plurality of grinding machines dedicated to either pencil-shaped wheels, flared or cup-shaped wheels. A few incorporate a machine tool design to allow the exchange of spindle assemblies and drive mechanisms to accommodate the physical orientation for either pencil-shaped wheels, flared or cup-shaped wheels. Most require the use of dedicated machines with physical orientation as capable for only pencil-shaped wheel grinding, only flared wheel grinding or only cup-shaped wheel grinding. In most if not all cases, the cam relief motion on the machine tools use one axis to provide the radial motion to form the helicoidal surface (i.e. cam relief) which limits the machine's flexibility.